Field of the Invention
As methods for joining two resin members together, ultrasonic vibration welding and heat welding can be primarily used. According to ultrasonic vibration welding, surfaces of two resin members, where the resin members are to be joined, are formed into such shapes as inducing concentration of stress. While maintaining the resin members in contact with each other under pressure, the resin members are vibrated by an ultrasonic wave from an ultrasonic wave generator so that frictional heat is generated at the surfaces to be joined. The surfaces to be joined are hence fused and joined together.
Known examples of heat welding, on the other hand, include heat welding, eddy-current electromagnetic induction welding and direct heat welding. In heat welding, mutually-opposing surfaces to be joined are entirely heated and fused and are then brought into contact with each other under pressure so that they are joined together. According to eddy-current electromagnetic induction welding, a resinous bonding piece added with electrically-conductive powder such as iron powder is arranged between surfaces to be joined, a high-frequency current is caused to flow through a coil arranged in a vicinity of the surfaces to be joined, and the electrically-conductive powder is heated by an eddy current generated through the coil so that the resinous bonding piece is fused to join the surfaces together. According to direct heating welding, a heating element is arranged between surfaces to be joined, the surfaces to be joined are fused by the heating element so that the surfaces are joined together including the heating element see Japanese Patent Application Laid-Open (Kokai) No. SHO 62-267125!.
However, ultrasonic vibration welding and eddy-current electromagnetic induction welding generally require expensive equipment and hence result in high production cost. Since surfaces to be joined are fused by frictional heat generated by vibrations in ultrasonic vibration welding, this method does not have much tolerance as to the configurations of surfaces to be joined, and its application is often limited to planar surfaces. Such planar surface configurations however have difficulty in assuring sufficient joint strength in a direction perpendicular to the joined surfaces (in other words, in a pulling direction, namely, in a direction that the joined surfaces separate from each other), that is, in a direction which is generally considered to be most important, although sufficient joint strength is available in a direction parallel to the joined surfaces because the parallel direction is a shearing direction of the joined surfaces. The term "surfaces to be joined" as used herein means surfaces of mutually-opposing two resin members, which are located at opposing positions and are used to join the two resin members together. The same surfaces may hereinafter also be called "joining surfaces" before joining and "joined surfaces" after joining.
Eddy-current electromagnetic induction welding and direct heat welding cannot avoid inclusion of a substance other than a resin material, such as iron powder or a heating element, between joined surfaces. Upon recycling the resin material, advance removal of the additive substance contained in the resin material, such as the heating element, is indispensable. This is certainly not favorable for recyclability. On the other hand, when surfaces to be joined are fused with a heating element kept in contact with the surfaces as in direct heating welding, fusion takes place centering around areas of contact with the heating element so that the fusion may not be achieved to sufficient extent where the surfaces to be joined are wide. Any attempt to arrange more heating elements with a view to overcoming this inconvenience, will however lead to a further deterioration in recyclability. Under the ever-increasing concern about environmental problems and resource reutilization in recent years, the recyclability of resin materials is an extremely important theme.